A 28-year-old female with a history of recurrent skin and soft tissue infections presents with a 4-day history of fever, malaise, and a rapidly enlarging, painful boil on her left thigh with surrounding erythema and warmth. Pus is aspirated and sent for investigation. Gram stain shows gram-positive cocci in clusters. Which investigation is most appropriate to determine if this isolate is methicillin-resistant and guide empiric antibiotic therapy?

Explanation

## Methicillin Resistance Detection in S. aureus ### Why Oxacillin/Cefoxitin Disk Diffusion is Correct **Key Point:** Oxacillin disk diffusion (or cefoxitin disk diffusion as a surrogate) is the CLSI-recommended screening method for detecting methicillin resistance (MRSA) in *Staphylococcus aureus* isolates and is the most practical, cost-effective investigation for guiding empiric therapy. **High-Yield:** Methicillin resistance detection is CRITICAL because: - MRSA requires vancomycin, linezolid, or daptomycin - MSSA responds to beta-lactams (nafcillin, oxacillin, cephalosporins) - Empiric therapy selection depends entirely on MRSA status ### Interpretation of Disk Diffusion Results | Test | Disk Used | Resistant Zone | Interpretation | | --- | --- | --- | --- | | Oxacillin disk diffusion | 1 μg oxacillin | ≤10 mm | MRSA (methicillin-resistant) | | Cefoxitin disk diffusion | 30 μg cefoxitin | ≤17 mm | MRSA (more sensitive than oxacillin) | | Vancomycin E-test | Vancomycin strip | MIC ≥2 μg/mL | Vancomycin-intermediate (rare) | **Clinical Pearl:** Cefoxitin disk diffusion is now preferred over oxacillin because it better predicts mecA gene presence and is more sensitive for detecting borderline oxacillin-resistant strains (BORSA). ### Why This Investigation Guides Therapy **Mnemonic: MRSA Therapy = VLD (Vancomycin, Linezolid, Daptomycin)** - **MRSA detected** → Use vancomycin (15–20 mg/kg IV q8–12h) or linezolid (600 mg IV/PO q12h) - **MSSA detected** → Use nafcillin or oxacillin (1–2 g IV q4–6h) or cephalosporin This is why oxacillin/cefoxitin disk diffusion is the investigation of choice — it directly determines which antibiotic class to use.